You description lacks self-consistency: adults look like human but children are born from fish-like animals; children look like fish but adults brood humanoid children in their mouth.

@L.Dutch The children are fish-like humanoids, and are born from fish-like animals, and grow into humanoid adults

@IchthysKing but where do the fish-like original parents come from?

@StarfishPrime That's what I'm asking

The question asks about a life-cycle, but you haven't supplied one. If you wanted an answer that completed the cycle by linking the final human-like forms with the original fish-like ones, then it would be nice if you could state it explicitly.

So, they have a fish stage, a human stage, then a fish stage - is that it, and you want some kind of justification for this?

Start again & try to lay it out comprehensibly this time, something like A produces > B produces > C, use bullet points if you have to, you might think you have laid it out clearly already but you haven't, I'm afraid its just a mess right now // the question I presume is something like where does A come from?

1. Large mammal-fish produce baby-fish (that they brood in their mouths until they're larger). // 2. Baby-fish metamorphose into human appearing aqua-folk (later in their life cycle when they're bigger). // Question: where do the mammal-fish come from? // Is that it? // I really shouldn't have to work back & forth through the question multiple times just to find out what it is & then have to ask if I've got it right, please sort it out.

If I have got it right ^ then this has to be closed as opinion based because the answer could be literally anything including any number of intervening lifecycles or even that they're a dead end like worker bees or ants & the mammal-fish reproduce among themselves entirely separate from the 'workers' // meantime I'm VTC as lacking clarity.

If it is like the Anaximander version of human evolution, should it include them clawing their way out from within the rough-clad fish stage?

It took me a couple of reads before I noticed the "xenogenesis" bit at the end. You're starting with Anaximander's 'theory of evolution'. In Anaximander's theory, the adults are humans ready to go off and make normal human babies. The fish was just a launch stage, never needed again. Are you looking for the origin of Anaximander's fish? And how much of Anaximander's story are you set on keeping?

I need to agree with @legio1 Anaximander's fish is a creation story, not a life cycle, and not based on biological science but elemental theory of the time. But the question could be restated to make a question that works.

@Pelinore Is my edit clearer now?

(a) Version #5 is a high concept question that should remain closed. Describing a cool creature then asking us to invent everything else about it is the essence of an HCQ. Please ask specific questions per the help center. (b) Version #5 is also inconsistent. You can't have an adult fish bear a "tadpole" that then morphs into a human without that human either birthing the fish (that in turn births the tadpole) or later morphing into the fish (that births the tadpole).

@JoinJBHonCodidact I've already developed most of the life-cycle, so this is far from a HCQ. On the inconsistency, what are you talking about? Is any question inconsistent if it isn't answered in the main question text?

Missing from life-cycle: the reproductive stage of their life. If they are born by a fish-life parent, in what stage of their life they transform from their human appearance into the fish-like form? Or are they able to reproduce and are sexually active before transforming in their human-like appearance, which is sexually sterile?

@AdrianColomitchi That's what the question is asking about

@IchthysKing It's an HCQ because you're NOT asking us about the life cycle, you're asking us how they could "survive and reproduce." That's inventing an entire biome.